1. Field of the Invention
The present invention pertains to a mobile terminal mounted in a high-speed mobile object and a wireless communication system including the same mobile terminal.
2. Description of the Related Art
At present, when accessing in real time desired information located on the Internet or the like from information terminals mounted in mobile objects like human beings, cars, and trains, it is mandatory to utilize wireless communication systems. As for wireless communications, cellular phones are generally used relative to telephone conversations and electronic mail and, further, the dissemination of the communication networks therefor has advanced across wide areas covering regions with about 90% or more of the domestic population in Japan. Consequently, cellular phones are valid with respect to movements over wide areas, and in addition, they are valid with respect to high-speed motion. Incidentally, as for cellular phones, telephone conversations while onboard a Shinkansen train are possible, i.e. they can be used even during high-speed movements at speeds of 200 km per hour or more.
Consequently, even in the case of accessing the Internet or the like from an information terminal mounted in a mobile object, the simplest method is that of utilizing a cellular phone and the communication network therefor. Even at present, Internet connection service is performed via cellular phone networks, and cellular phones themselves are in the process of becoming information terminals capable of Internet connections.
In recent years, a next-generation information provision service meant for motor vehicles and combining telecommunications and informatics, called telematics, has gradually moved into the spotlight, and more recently, commercial service has also been introduced within Japan. In these telematics services, the communication network of a cellular phone is put to practical use and in order to receive a service, it is common to pay not only a service fee to the telematics operator but also communication charges to the wireless communication operator. However, when it comes to telematics users, the culture of paying charges for the communications as well has not yet got established, so this has become one primary obstructing factor for the dissemination of telematics.
Incidentally, as a representative example of a wireless system in which it is not necessary to pay a communication charge, there is the Wireless LAN (Local Area Network). Moreover, if this Wireless LAN is used, the construction of a private wireless network is possible. That is to say that it is possible, by means of a Wireless LAN, to implement a very versatile telematics service capable of letting anybody enter anytime and without charges. However, the greatest issue in the case of applying this Wireless LAN to telematics resides in the fact that communication with a terminal on a high-speed moving object like a motor vehicle or a train is difficult because from the outset, in Wireless LAN, the speed of motion of the mobile terminal was only imagined to be at most approximately the speed of a pedestrian.
The reason why the same communication turns out to be difficult is that, in the case of Wireless LAN (IEEE 802.11 Specification), the area in which communication is possible has a range limited to at most several hundred meters, and in addition, that it is not possible to restrict the number of frequency channels used in that communication to one and that an excessive time is required for the frequency channel search for the purpose of setting up a communication link. This will be explained by FIGS. 11A and 11B. Here, FIGS. 11A and 11B are diagrams showing a general example of a conventional frequency channel search, FIG. 11A being a diagram showing the relationship between frequency channel and time during the same frequency channel search and FIG. 11B being an example of search sequence data.
If e.g. a mobile terminal mounted in a motor vehicle enters the communication area of some base station of the Wireless LAN, a frequency channel search is carried out in order to set up a communication link with that base station. As shown in FIGS. 11A and 11B, in the case where e.g. frequency channels F1 to F14 are allocated in the communication band of the Wireless LAN, the mobile terminal searches the same frequency channels by number from F1 to F14. Then, when it has been possible to confirm the frequency channel used by the base station thereof, a communication link is established and transmission and reception of data is carried out.
On the other hand, in the case it took e.g. 2 s for the search of one frequency channel, in the worst case, it turns out that a time of 28 s is required until the establishment of a communication link. In that case, since, when the mobile terminal is moving at a speed of 60 km per hour, the same mobile terminal moves 470 m in 28 s, it ends up getting out of a normal Wireless LAN area, so it has not been possible to carry out transmission and reception of data.
As against this, in JP-A-2002-57615 (Paragraph 0021 to Paragraph 0035, FIG. 1 and FIG. 2), there is disclosed a communication system designed to shorten, by increasing the frequency of occurrence of searches of a specific frequency channel allocated to a service for which it is necessary to handle high-speed motion, the frequency channel search time, i.e. the time until the establishment of a communication link, and ensure a data transmission and reception time, in the case of receiving such a service.